1. Field of the Invention
This invention relates to automatic test equipment for testing integrated circuits and the control of signal timing apparatus in such equipment. In particular, the invention relates to the control of apparatus for delaying electrical signals by desired time intervals in automatic test equipment.
2. Description of the Prior Art
Automatic test systems for the testing of integrated circuits are well known and employed throughout the semiconductor industry. In such test systems a semiconductor device under test is subjected to patterns of stimuli signals, and the resultant output conditions are monitored and analyzed to determine the functionality and/or quality of the device being tested. As integrated circuit technology advances, the manufacturers of such automatic test systems must provide the capability of testing increasingly complex, very high speed circuits with a high degree of precision. In such test systems to assure the validity of the test results, the timing system must cause input signal transitions and output strobe signals to occur precisely at pre-programmed times at the device under test. A further complicating factor in such systems is that the signals being supplied to, and received from, the terminals of the device being tested, travel over many different paths through the system circuitry. Because the different paths have different propagation delays, the propagation delays must be corrected to assure that the patterns of test signals supplied, and that the output signals detected, are validly interpreted.
In prior art test systems the propagation differences were compensated for by the use of manually adjustable potentiometers. Adjustment of these potentiometers was a time consuming and expensive task.